CHAPTER III 
Ancient Volcanoes ; Proofs of their existence derived from the Nature of the Hocks erupted 
from the Earth’s Interior. A. Materials erupted at the Surface — Extrusive Series, 
i. Lavas, their General Characters. Volcanic Cycles, ii. Volcanic Agglomerates, 
Breccias and Ttrffs. 
The materials brought by volcanic action from the earth’s interior Imve 
certain common characters which distinguish them from other constituents 
of the terrestrial crust. Hence the occurrence of these materials on any 
part of the earth’s surface affords convincing proofs of former volcanic 
eruptions, even where all outward trace of actual volcanoes may have been 
effaced from the topograpliical features of the ground. 
Volcanic products may lie classed in two divisions — 1st, Those which 
have been ejected at the surface of the earth, or the Extrusive series ; and 
2nd, Those which have been injected into the terrestrial crust at a greater 
or less distance below the surface, and which are known as the Intrusive 
series. Extrusive rocks may be further classified in two great groups 
(i.) The Lavas, or those which have been poured out in a molten condition 
at the surface ; and (ii.) The Fragmental Materials, including all kinds of 
pyroclastic detritus discharged from volcanic vents. 
Taking first the Extrusive volcanic rocks, we may in tlie present chapter 
consider those characters in them which are of most practical value in the 
investigation of the volcanic phenomena of former geological periods. 
i. LAVAS 
The term Lava is a convenient and comprehensive designation for all 
those volcanic products which have flowed out in a molten condition. They 
differ from each other in composition and structure, but their variations are 
comprised within tolerably definite limits. 
As regards their composition they arc commonly classed in three di-vi- 
sions — 1st, The Acid lavas, in which the proportion of silicic acid ranges from 
a little below 70 per cent upwards; 2nd, The Intermediate lavas, wherein 
the percentage of silica may vary from 55 to near 70 ; and 3rd, The Basic 
lavas, where the acid constituent ranges from 55 per cent downwards. 
Sometimes the most basic kinds are distinguished as a fourth group under 
the name of Ultrabasic, in which the percentage of silica may fall below 40. 
